A human guinea pig in the brown fat lab

I spent a day finding out first-hand what it’s like to take part in an experiment aimed at devising new treatments for obesity. I’m a “control”, providing baseline metabolic data against which the two experimental groups can be compared. But it involves a lot more than filling in a questionnaire and twiddling your thumbs, as I found out. Andy Coghlan

Not a death ray, thank goodness, but a Dual-Energy X-ray Absorptiometry (DXA) scan. It measures the relative amounts and distribution of fat, lean and bone tissue, which are important for working out how efficiently you burn energy. The scanner gradually runs over your entire body, revealing what's inside by passing very low-energy X-rays through it – equivalent to the amount of radiation you'd absorb from cosmic rays during a flight in Europe. Very restful, as the scanner makes gentle humming sounds as it trundles down the bed.

And here's what it revealed. The bones in my skeleton are exactly the correct density for someone of my age, which is good news. Even better, you can see that my fat is pretty evenly spread across my abdomen and my upper legs, with about 22 per cent of my body fat in each of these bodily compartments.

By contrast, through middle-age spread, many men of my age have around 30 per cent of their body fat over the tum, which is bad news: as it's close to your main visceral organs, it can mess up fat metabolism, leading to type 2 diabetes, obesity and an increased risk of cardiovascular disease.

The graph shows that I've got about 6 per cent less fat overall than a typical man of my age, which is also something of a relief, given my predilection for one too many beers.

Escape from the Bodpod. This apparatus, which looks like some kind of submersible for probing the Mariana trench, measures the relative amounts of fat and lean tissue in your body. It works it out from the amount of air you displace and your height and weight. Through subtle pressure differentials created as air flows over fat and lean tissue, it calculates how much you have of each. But for it to work, as much of your flesh has to be exposed as possible, and your hair has to be covered: hence the cap and the general lack of apparel (sorry about that unwelcome spectacle). Feels very odd as the air flows over your body, making your flesh wobble slightly!

Time to measure how fit (or not) I am. First I'm fitted with a belt carrying sensors and electrodes that will measure how efficiently I burn oxygen. Then the treadmill. I've never been on one of these before (except the one called the news department at New Scientist). It starts off at a stroll, builds to a canter and ends in a heart-pounding frenzy, not helped by the clippy thing on my nose that forces me to breathe through my mouth into the attachment. But I manage to survive, even when it feels like I'm in the Grand National. Metabolic assistant Laura Watson is pointing approvingly to the chart, telling me my peak oxygen consumption is normal. Or something. I'm too tired to take it in!

A human-sized calorimeter. Remember those experiments at school with calorimeters to measure the specific heat capacity of water and so forth? Well, here they're treating me like a fossil fuel, seeing how much I burn overnight by turning oxygen and food into carbon dioxide that I breathe out. From that, they can work out how efficiently I've burned my fuel, and how much energy I've generated.

Some other participants are kept in the cold. Presumably they will burn off more than me to keep warm. And if the spicy capsule given to the second experimental group works as hoped, so will those participants. Because absolutely everything I produce is saved and measured, including all my urine, I freak out about whether breaking wind will make the room uninhabitable by dawn. But later I'm told it's all part of the experience. Well, here we go. I'm in for the entire night. Nice view of the car park if I get bored.

This is the bit where the little alien bursts out of my tummy and rampages through the unit. No sorry, that was a dream. In reality, I've just woken from my slightly fitful night's sleep in the calorimeter, and they measure my resting respiration rate. I wear this attachment for a good 20 minutes. Still a bit dozy, there's a lovely white-noise hum that's extremely relaxing – undoubtedly the highlight of the whole experience. I beg them not to take it off till noon, but after 20 minutes of bliss it's whipped off and my duties as a control are complete. Now I can't wait to find out the result of the experiment, although one satisfying aspect is that my data will be used in several other experiments at the unit as well.